In semiconductor manufacturing wafers are sequentially treated in a plurality of process steps during the production process. With increasing density of integration the quality requirements in respect of the structures formed on the wafers increase. In order to monitor the quality of the formed structures during the production process and to potentially find defects, the requirements as to quality, precision, and reproducibility of the elements handling the wafer and of the process steps are correspondingly high. This means that in the production of a wafer with a plurality of process steps and a plurality of layers of photoresist to be applied a reliable and early detection of defects is particularly important. In the optical detection of defects also the systematic errors due to variations of thickness of the application of photoresist to the semiconductor wafers need to be detected reliably and reproducibly. Therein it shall be possible to mark only those locations on the semiconductor wafer, which actually contain an error, which eventually has a negative impact on the functionality of the respective DIE or chip.
The international patent application WO 2010/124081 A1 discloses a method for optimizing an optical inspection and production process of semiconductor elements. Herein preferentially color images of the objects to be examined are recorded. From these images a plurality of representations in different color spaces is generated. Each of these representations and the channels, or data, corresponding to these representations, are analyzed. Therein the analysis can be carried out separately or in combination with other channels, in order to find out which representation or which combination of representations, channels, or combination of channels or data or combinations of data, provide the most adequate representation, in order to therewith carry out the analysis with an optical inspection algorithm. Therein the process may be automatized with respect to the generation of the representations of the image and/or the individual or multiple analysis.
The German patent DE 10 2007 039 982 B3 discloses a method for the optical inspection and visualization of the optical measurements obtained from disc-shaped objects. For this first at least one image of the at least one disc-shaped object is recorded. From the at least one image recorded a plurality of optical measurements is generated. Then a result image is generated, wherein to an area of the surface of the disc-shaped object the optical measurements of which are within a pre-defined interval, a color or brightness value is assigned. The color or brightness value is chosen from a pre-defined palette. Eventually at least one imaging parameter is varied in dependence on the recorded and evaluated optical measurements and/or in dependence on a visual check of the result image by an operator.
The German patent DE 10 2006 042 956 B4 discloses a method for optical inspection and visualization of optical measurements obtained from disc-shaped objects. At least one image of the at least one disc-shaped object is recorded. A plurality of optical measurements is obtained from the at least one recorded image. Then a color value is assigned to each optical measurement. Therefrom a result image is generated, wherein a color value, selected from a pre-defined palette, is assigned to an area of the surface of the disc-shaped object the optical measurements of which are within a pre-defined interval.
The German patent application DE 103 07 454 A1 discloses a method, an apparatus, and a software for the inspection of surfaces of a semiconductor substrate. Furthermore a method and an apparatus for the production of a structured semiconductor substrate using such a method or such an apparatus, respectively, are disclosed. In the method an image is recorded for the optical inspection of the surface of a semiconductor substrate. The image comprises a plurality of pixels with at least three corresponding intensities of different wavelengths, referred to as color values. By a transformation into a color space spanned by the intensity and color coordinates, a frequency distribution of pixels with equal color coordinate values is calculated from the color values. The frequency distribution calculated in this way is used for a comparison with a second frequency distribution calculated in a corresponding manner or with a quantity derived therefrom. By this method only the statement can be made that there is a defect on the surface of the disc-shaped substrate. No statement regarding the location of the defect on the surface of the disc-shaped substrate can be made.
Macroscopic images of semiconductor wafers or of surfaces of disc-shaped substrates show that the homogeneity of the layers changes radially. In particular during the application of resist changed homogeneities arise in regions distant from the center of the wafer. If, as has been done up to now, a uniform sensitivity is used for the entire radius of the wafer for the evaluation of images of the wafers recorded, it happens that the deviations at the edge are detected always, defects in the interior (close to the center of the wafer), however, are not detected. If a high sensitivity is chosen in order to surely detect defects in homogeneous regions, then in the edge regions erroneous detections occur increasingly, because the inhomogeneous edge regions are not always to be classified as defects. In order to avoid this, the edge regions may be excluded entirely. Then, however, no real defects are found there. If a low sensitivity is chosen, then no erroneous detections occur any longer, defects in the homogeneous regions, however, cannot be found then.
The German patent application DE 103 31 686 A1 discloses a method for evaluating images recorded from wafers or other disc-shaped objects. After recording the image of at least one reference wafer the radial distribution of the measurements of the reference wafer is determined and displayed on a user interface as a radial homogeneity function. A sensitivity profile with radial dependence is changed taking into account the measured radial homogeneity function of the reference wafer. At least one parameter of the sensitivity profile is varied, so that a learned sensitivity profile is determined visually from the comparison with the radial homogeneity function. This method does not show an image of the entire wafer, based on which the image or the images are evaluated with respect to the defects, either.
The U.S. Pat. No. 7,065,460 discloses an apparatus and a method for the inspection of semiconductor elements. With the apparatus the electrical properties of the semiconductor product are examined. For representation on the display different colors are assigned to the results of the measurements.
Many prior art methods for inspecting the surface of a disc-shaped object for defects are known. Therein a wide variety of algorithms are used in order to visualize the defects in the images obtained from the measurements. Many of these methods, however, are not suitable for reliably detecting the defects. Eventually it depends on the experience of the operator to actually classify a defect found as a defect.